[PATCH v10 18/37] mm: page_alloc: use aliasing checks instead of user_alloc_needs_zeroing

[Michael S. Tsirkin]

Replace user_alloc_needs_zeroing() with the direct aliasing checks
(cpu_dcache_is_aliasing() || cpu_icache_is_aliasing()) in the
post_alloc_hook aliasing guard.

user_alloc_needs_zeroing() includes a !init_on_alloc term that
means "allocator didn't zero this page."  But in this guard's
context (!zeroed && !init && __GFP_ZERO), we already know the page
is zero; init incorporates init_on_alloc via want_init_on_alloc().
The only question left is whether the cache architecture needs
the data re-zeroed through a congruent mapping, which is purely
cpu_dcache_is_aliasing() || cpu_icache_is_aliasing().

On non-aliasing architectures with init_on_free=true and
init_on_alloc=false, this avoids a redundant re-zero of an
already-zero page.

Note on PowerPC: PowerPC overrides clear_user_page to call
flush_dcache_page after clear_page, but on freshly allocated
pages PG_dcache_clean is already clear (cleared by
__free_pages_prepare), so flush_dcache_page is a no-op.
Skipping this here thus has no effect.

Signed-off-by: Michael S. Tsirkin <mst@xxxxxxxxxx>
Assisted-by: Claude:claude-opus-4-6
---
 mm/page_alloc.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 45e824b1ec75..edfc83571985 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1880,7 +1880,7 @@ inline void post_alloc_hook(struct page *page, unsigned int order,
 	 */
 	if (!zeroed && !init && (gfp_flags & __GFP_ZERO) &&
 	    user_addr != USER_ADDR_NONE &&
-	    user_alloc_needs_zeroing())
+	    (cpu_dcache_is_aliasing() || cpu_icache_is_aliasing()))
 		init = true;
 	/*
 	 * If memory is still not initialized, initialize it now.
-- 
MST